Real Aperture??

<p>Ok I was looking for some information about waht lens would be best to use for shooting meteor showers. I have the Nikon 35 1.8, and the Tokina 10-17 Fisheye. Ive been trying to work with the fisheye and had some fair results but then I came across this comment.</p>

<blockquote>

<p>However, also use largest <em>real</em> aperture you can. Stars (and meteors, for that) are point sources, and as such the real aperture, in mm, is more important than focal ratio, this dimensionless 1/something parameter.</p>

</blockquote>

<p>Being self taught I dont know alot of the indepth lingo so my question is because i cant seem to find an answer.... what is a REAL aperture? and Real in comparison to what? Thanks</p>

<p>-Michael Palozzola</p>

 

<p>It means use the lens with the largest physical opening at the aperture, regardless of what the f-stop says. For instance, the real or physical aperture on a 500mm f8 lens could have a larger diameter than that of a 16mm f2.8 lens.</p>

<p>I think they mean to say the largest opening you can, rather than an f-stop number. The f-stop is a ratio of the aperture to the size of the lens. So an f11 with your 35m is not the same size opening as it would be on a 200mm. So, to get the largest REAL aperture, as he is suggesting, you need to figure out which lens, at it's widest f-stop will give the biggest opening.</p>

<p> </p>

<p>Don't you hate it when people post while you are typing, and you end up repeating with they say ?</p>

<p> </p>

<p>So basically that would correspond ruffly to my largest lens like my 35mm 1.8 which has a 52mm barrel probably wouldn't be as useful as something with a 65 or 77mm barrel? And possibly another stupid question? how does that "real aperture" effect the image being taken? would an image taken on the 35mm at f/5.6 be different from an image on a wider "real aperture" at the same f/ with the same shutter time?</p>

<p>Michael, I'm not sure what the article was trying to say. I can't infer their point from the small excerpt that you've posted. However, let's talk about apertures.</p>

<p>Let's say that you have two lenses, a 50mm f/1.4 and a 100mm f/4. Let's calculate the maximum aperture for each lens. Remember that f is the lens' focal length. In this case, neither lens is a zoom lens, so their focal length never changes.</p>

<p>Aperture (lens 1) = 50 mm / 1.4 = 35.7 mm</p>

<p>Aperture (lens 2) = 100 mm / 4 = 25 mm</p>

<p>So, lens 1 has a larger maximum aperture even though lens 2 has a longer focal length (100 mm).</p>

<p>You can make these same calculations for zoom lenses, but it's a more complex combination. First, you need the actual focal length that you're using at the moment. - Read this from the lens barrel or take a photo and read it on your camera's LCD screen. - Next, you need the actual f-stop. Some zoom lenses vary their maximum aperture over the zoom range. </p>

<p>Let's say that you have a zoom lens called a 12-24mm f/3.5-4.5. The focal length can be anywhere from 12 to 24 millimeters. The maximum aperture can be anywhere from f/3.5 to f/4.5 depending on your focal length. Again, the best way is to shoot a photo and read this information in the EXIF data displayed in your camera's LCD screen.</p>

 

<p>So does this mean the exotic 300mm f2 (150mm aperture) is the King of Real Apertures in the wider-angle (a relative term) of big, bright lenses? The 600mm f4 gives the same Real Aperture but with a considerably narrower field of view. I know Nikon had a 1000mm f6.3 and Zeiss a 1000mm f5.6, but they are not exactly holdable even with a monopod; mind you 16lbs for the f2 is more than most mono-pods can handle! Even the new 200mm f2 is not going to cover much sky and the chances of catching a meteor are very small. I suppose if you have bottomless pockets you could make an array of 4 x 3 to cover about 45 degrees of sky and mosaic the results. I believe they do this somewhere in searching for asteroids, looks like a 'hedgehog' of white/grey Canon lenses I seem to remember.</p>

<p>In the simplest terms, the real aperture is the diameter of the opening that lets in light at the front of the lens; the diameter of the front element. Not the barrel diameter, not the filter diameter.<br /> The advice is given because the larger the diameter, the more photons will enter the lens for a given time. In astronomy, aperture is king.</p>

<p>Ok i think i get what the "real aperture" is but what I still don't get is would an image taken on the 35mm at f/5.6 be different from an image on a wider "real aperture" for instance maybe the 24-70mm @ 35mm f/5.6 (it is a larger lens so presumably has a larger "real aperture") but shot with the same shutter time? Would the resulting image be different?</p>

<p> </p>

<p>Meteors are NOT point sources. Only stars are. Meteors have "dimension" just like the planets.</p>

<p>Use the 35mm 1.8G.</p>

<p>even over the 10-17mm f/3.5-4.5 Fisheye?</p>

<p><em>...would an image taken on the 35mm at f/5.6 be different from an image on a wider "real aperture" for instance maybe the 24-70mm @ 35mm f/5.6 (it is a larger lens so presumably has a larger "real aperture") but shot with the same shutter time?</em><br>

These two will give you the same exposure. By stopping down the lenses, you have made the apertures the same regardless of the diameter of the lens <em>when wide open.</em><br>

The 'real aperture' in the original post means the aperture wide open.</p>

<p>sooo by that rational wouldn't it just be to use the fastest lens possible? Also what I've heard from most people they shoot around f/8-11 on longer exposures for meteor photorgraphy, so why would you need that "real aperture" ... i guess im still confused lol</p>

<p>fastest & widest lens possible actually but what about like<br>

35mm @ 1.8 vs 50mm @ 1.4 ... what is the benefit to the f/.4 faster? is it worth losing the FoV of the 15mm?</p>

<p><strong>Here is the full context of the statement in a thread about photographing meteor showers:</strong></p>

<p>"Hi,<br /> it depends on he equipment and on the character you'll want for the final images. I suppose you do not have a mounting that rotates the camera against the earth, so that the stars would be spread out to streaks on a more that 15 to 30 second exposure, depending on focal length. So, here's some advice:</p>

<ul>

<li><strong>Lens</strong> : Use a short focal length, to maximize field of view. However, also use largest <em>real</em> aperture you can. Stars (and meteors, for that) are point sources, and as such the real aperture, in mm, is more important than focal ratio, this dimensionless 1/something parameter. I would go for a 50mm, used at 1/1.4 or less (meaning 36mm or more real aperture), which gives a good trade-off between field size and aperture IMHO.</li>

</ul>

<ul>

<li><strong>Camera settings</strong> : If digital (I just suppose), get a <em>huge</em> memory card and take exposures of a few to a few ten seconds at most, until the card is full, basically. The exact time depends on the brightness of the sky at your location and the sensor noise and dark-current behavior. The average fall time is a fraction of a second, so you are not going to lose anything from the meteor, but depending on your location you minimize background stray light and smearing of stars into streaks (unless you want that, of course, but this makes it harder since it is another type of optimal settin). Use at high ISO, even if the shutter is open for seconds the exposure time for a meteor will more likely be of the order of 1/100s per pixel, then it has moved to the next pixel. Higher ISO beats reduction of background, rather reduce exposure time if background is an issue.</li>

</ul>

<ul>

<li><strong>Postprocessing</strong> : Identify the frames with meteors on. Since you most likely have not tracked the stars (see the mounting thing above), you'll have to shift/rotate/etc those frames to be able to stack them (btw, at least crudely follow the field with your camera by tracking manually every 15 minutes or so). Same procedure as for combining hand-held panorama sots, just on a single field. Then combine them, no idea what the algorithm would be called in your software, but in principle it would be something like "only superimpose brighter pixels". If you feel you have too much noise, too few stars, chose some of the frames with no meteors, construct a nice background image for the field, and then superimpose only the meteors/bright stars from the shooting star combined image.</li>

</ul>

<p>Good luck then, and although Lyrinds are not spectacular, it is oing to be a good training run for the Perseid and Leonid showers lather in the year."</p>

<p><strong>I believe the point of the comment is to keep your exposures as short as possible to eliminate star trails when photographing meteor showers. For other astrophotography, unless your lens is very sharp wide open, then stopping it down a little bit would be a good idea. With a good lens/camera combination, you can still keep exposures short enough to prevent star trails. I'm not sure about the astrophotography experience of the poster - there are no meteor or star photos on his website. Some of the statements made in the 'camera settings' paragraph are also quite confusing; mostly the part about a meteor only traveling one pixel in 1/100 of a second. f/2.8 - or whatever - on any lens should let (nearly) the same amount of light in the camera, so 'real aperture' measurements seem to be irrelevant.</strong></p>

<p>Try shooting a few 35/1.8 test shots at f/2, f/4 and maybe f/5.6. The main difference you'll find is the "blackness" of the background sky. If you are in a truly dark area, the larger apertures (smaller f/number) will allow stars and meteors to be brighter. But if your area has light pollution, the larger apertures will give you a greyish background on long exposures. You can test on a plain old starry night, not just on a shower night. Shoot the Big Dipper and/or the Gemini-Orion areas to get an idea of how bright the actual stars/meteors will be at each setting.</p>

<p>Keep in mind any Moon past about a Quarter Moon will lighten the sky just like light pollution does, and will make any particular-magnitude star or meteor track appear dimmer.</p>

<p>Larger apertures will allow shorter exposures, so star trails will be minimized. But long exposures with some star trails and a meteor zipping through your frame makes for a nice shot, too. It all depends on what you're after. Like all of photography, the best way to learn what different settings result in is to shoot shoot shoot. Basically, you want bright stars on a black background (some of this can be improved in post processing). With digital, keep the exposures below 30 seconds or so to minimize noise.</p>

<p>If you find f/4 pleases you, shoot the 10-17 Fish to get a wider-field capture. Wider is certainly better when trying to catch meteor trails.</p>

<p>Well I would love to go out on any random starry night to shoot and work out all the kinks but... I live in East South Florida (a very WELL lit area with Miami just south of me and Boca/West Palm to the north). So on Meteor Nights I've been going out west to the Immoklee area on the other side of the state. For those of you familiar with West South FL, I'm looking for a better site if you know any because my site though dark for S.Fl it does still get a lot of glow from nearby farms and Immoklee itself(and Naples I'm assuming, they are both basically NW of my spot). So if anyone knows anything south of Immoklee like any good locations heading south on 29 or something I'd love to know? But yeah that is a far drive and about half a tank o gas to just go fiddle with my camera settings on any random night. So, I am trying to learn as much as possible "book knowledge" before I go out again in Dec to shoot the Geminids. (Or at least have a specific SET of settings I want to go out there and fiddle with and try out.)<br /> I've only worked with the Tokina 10-17mm f/3.5-4.5 -fisheye on my previous two attempts at this meteor photography because I didn't own the 35mm f/1.8 in august for my first shower, the only other lens I had access to at that time that is wider then my Micro-Nikkor 60mm f/2.8 is my fisheye and my father's Nikkor 24-85mm f/3.5-4.5 (which I don't like because imho I find that lens to be unacceptably soft). Then this past time I went out I didn't even think about my new little fast piece of glass, but seeing how in the sharpness test I did I found the 35mm f/1.8 to be an acceptably sharp at 1.8-3.5 and razor sharp from 3.5-11, I guess it couldn't hurt to throw that one on this time for a bit and see how it looks with that FoV. Also I've never worked with anything as faster then f/3.5 for starry night conditions, so ill have to try it and see because as has been said before It all depends on the preferences of the photographer and personally I don't know my preferences having not really achieved a picture I am very proud of yet. So slowly I am just building a list of things I do NOT prefer :)<br /> But in all honesty I think they really just need to make a 10mm f/>1.8 for this... but then again we waited how long for them to put out a "standard prime" (50mm equivalent) on the DX format that was a strong enough lens to warrant discarding the occasionally faulty Sigma 35mm.</p>

<p>edit: WOW.. Im sorry I didn't realize I was writing a book.</p>

<p>Michael....</p>

<p>Sorry for my delay. I was away over the weekend.</p>

<p>Go to the beach and look east. There's no population offshore, and there's got to be some sort of park or barrier island reserve north or south of you to minimize light around you. Orion will come up in the east, so placement couldn't be better. Be sure your target is at least 20 degrees above the horizon to avoid the worst of the marine layer. It won't be perfect, but it will give you an idea of what your settings are doing to your results. And it will be good practice for your west coast dark site.</p>

 

<p>The problem is with the cities behind me to the North and South, the glow extends into the open ocean, granted it is Better facing open water, but there is still a heavy glow which I believe would effect my camera settings substantially. Id probably have to go way above west palm to get a truly dark beach. Also it doesnt help that about a mile out there is almost ALWAYS a line of Cruise ships at night which are lit up like Christmas trees</p>

<p>"Try shooting a few 35/1.8 test shots at f/2, f/4 and maybe f/5.6. The main difference you'll find is the "blackness" of the background sky. If you are in a truly dark area, the larger apertures (smaller f/number) will allow stars and meteors to be brighter. But if your area has light pollution, the larger apertures will give you a greyish background on long exposures."<br>

-<br>

I disagree. Use the maximum aperture you can live with, and a high ISO (1600 is a good start), then adjust your exposure time so that the background is correctly exposed (ie not to light - you want some contrast or meteors will be hard to see). If your exposures are too short at this point, drop down to ISO 800. You should always take some test shots of your sky first.<br>

-<br>

Here is some software that will help you choose the most efficient lens (<a href="http://www.analemma.de/download/meteor.zip">Meteor Capture Calculator</a> ):<br>

http://www.analemma.de/english/download.html<br>

-<br>

Sorry, I can't offer any suggestion as to how you can eliminate the light pollution (apart from driving away from it). Good luck!</p>

<p>For some reason I can't edit my previous post!?<br>

-<br>

Anyway, I just wanted to add, although wider angles will cover more sky, most meteors you catch will probably be very small compared to the size of the whole frame. Unless you catch lots of them, and stack, the result probably won't be too impressive.<br>

-<br>

Longer focal lengths on the other hand (35-50mm is ideal IMHO) will give you a better balance between sky coverage and the size of the streak in the photograph. As you mentioned earlier, a battery of short FL lenses is considered ideal for catching meteors:<br>

http://web.archive.org/web/20020610191659/http://delpsurf.www.cistron.nl/haas_array.html<br>

http://www.robertreeves.com/cluster.htm<br>

-<br>

Here's the battery you mentioned earlier:<br>

http://www.open2.net/blogs/scitechnature/index.php/2009/07/31/what-is-superwasp-telling-us?blog=7<br>

http://www.spacedaily.com/reports/Astronomers_Catch_Planet_By_Unusual_Means.html</p>